1. Field of the Invention
The present invention relates to a developing apparatus for developing an electrostatic latent image formed on an image bearing member with a developer, adapted for use in an image forming apparatus such as a copying apparatus or a printer.
2. Related Background Art
An image forming apparatus utilizing an electrophotographic process is utilized, in addition to a copying apparatus, as an electrophotographic image forming apparatus such as a printer or a facsimile apparatus. Particularly in a printer or a facsimile, for the purpose of reducing the dimension of a copying apparatus portion and of facilitating maintenance, a unit formation is progressing in a developing unit constructed around a developing apparatus and a drum unit constructed around an electrostatic latent image bearing member (image bearing member), and a process cartridge integrating these units is employed frequently.
Also such process cartridge often adopts a one-component developing method which is advantageous for achieving a compact structure.
The one-component development method utilizes a one-component developer (toner), and is executed in a developing apparatus having a developer regulating member such as a developing blade and a developer carrying member such as a developing roller. In the developing apparatus for executing the one-component development method, the developing roller is positioned opposed to an image bearing member such as a photosensitive drum, on which an electrostatic latent image to be developed is formed, scoops up the toner from the interior of the developing apparatus and carries it on a roller surface by a rotary motion to a development area. Along the periphery of the developing roller and between a portion for scooping up the toner and a developing area, there is formed a portion opposed to a front end of the developing blade. In such opposed portion, toner particles are given a charge by a friction between the developing blade and the toner particles and by a friction between the developing roller and the toner particles, and are coated as a thin layer on the developing roller, whereby the toner is carried to the developing area where the developing roller and the image bearing member are mutually opposed to develop the electrostatic latent image on the image bearing member, thereby obtaining a visible toner image.
Different from a two-component developing method requiring carrier particles such as glass beads, iron powder or ferrite, the one-component developing method can dispense with such carrier particles and can achieve a small and lighter structure in the developing apparatus itself. Also the two-component developing method requires, in order to maintain a constant concentration of the toner in the developer, an apparatus of detecting a toner concentration and replenishing a necessary amount of the toner and leads to a larger and heavier structure of the developing apparatus, while the one-component developing method does not require such apparatus. For these reasons, the one-component developing method is advantageous for achieving a smaller and lighter structure.
In a full-color developing apparatus, it is not desirable, from the standpoint of color reproducibility, to include a magnetic powder which is generally colored in a full-color developer as a one-component developer. For this reason, a non-magnetic toner is widely employed as the developer.
On the other hand, there are required a printer and a copying apparatus capable of printing at a more printing speed. For meeting such requirement, an increase in the process speed is an issue to be considered, and, within an image forming process, a matching of a fixing apparatus and a toner in a fixing step, which is executed for fixing the toner on a recording medium in a step after a developing step.
Also in the fixing step, there is desired an improvement in usability such as a reduction of an electric power consumption and a quick start capability. In consideration of such situation, there is proposed a fixing apparatus of a film heating type, having a low heat capacity.
In the fixing apparatus of such film heating type, a nip portion is formed by pinching a heat-resistant film (fixing film) between a ceramic heater constituting a heating member and a pressure roller serving as a pressure member, and a recording medium bearing an unfixed toner image, to be fixed, is introduced between the film and the pressure roller in such nip portion and is conveyed together with the film, whereby, in the nip portion, the heat of the ceramic heater is given to the recording medium across the film and the unfixed toner image is fixed to the surface of the recording medium by the heat and pressure of the nip portion.
The fixing apparatus of such film heating method is characterized in that an apparatus of on-demand type can be constructed by employing members of a low heat capacity for the ceramic heater and the film, and by energizing the ceramic heater as the heat source to a predetermined fixing temperature only during an image formation of the image forming apparatus thereby providing advantages of a short waiting time from a turning-on of the power supply of the image forming apparatus to a state capable of executing an image formation (quick starting property) and a significantly smaller electric power consumption in a stand-by state (power saving).
However, the fixing apparatus of such film heating type is insufficient in a heat amount as a fixing apparatus of a full-color image forming apparatus or a high-speed image forming apparatus requiring a large heat amount for image fixing, and causes drawbacks such as a defective fixation, an unevenness in luster (gloss unevenness) in the fixed image and a toner offsetting, for which further improvements are desired.
As a result of detailed investigation with an image forming apparatus equipped with a fixing apparatus of film heating type, the present inventors have found that an image defect such as an unevenness in the gloss or an offsetting as mentioned above tends to be caused at a high-speed printing, regardless of the pressure in the fixing nip portion. Such phenomena are particularly conspicuous in case a thick paper (with a basis weight of 105 g or more) is employed as a recording paper.
For suppressing such phenomena, there is known a method of reducing a viscoelasticity of the toner, but such method, in a high-speed printing, causes an image defect by a fused toner bonding onto the surface of the developing roller, and various countermeasures thereto are being desired.
Also for enabling an image fixing with a low energy and presenting a stain caused by an offsetting phenomenon, there is known a method, as described in Japanese Patent Application Laid-Open No. H09-311499, of defining a temperature and a storage modulus of a one-component developer.
Also for matching with an oilless fixing operation with a Teflon roller, there is known a method, as described in Japanese Patent Application Laid-Open No. H06-59502, of defining a temperature and a storage modulus of a developer.